In NTSC, PAL or a similar television signal, two synchronization "pulses" are provided for synchronization at the receiving unit. One pulse, known as the "vertical synchronization pulse," indicates the beginning of a field.sup.1. The other pulse, known as the "horizontal synchronization pulse" indicates the beginning of a horizontal line. By tracking these pulses, the receiving unit is provided a time basis to align the image from field to field, and from line to line. FNT .sup.1 In NTSC or PAL television signals, the image is made up of two interlacing fields. The odd field is formed by the odd horizontal lines and the even field is formed by the even horizontal lines. The fields are provided alternately.
In addition to broadcast television, television signals are received from many sources, including cable television, video cassette recorders (VCRs), and laser disk players. (Television signals are also referred to as video signals.) However, the reliability as a time basis in a video signal received from any one of these sources varies with the technology used at the source. In particular, broadcast television provides the most reliable time basis. By contrast, a video signal from a VCR may provide synchronization pulses which locations vary over a relative large range, thereby making the task of tracking such video signal relatively difficult.
In a VCR, the video signal is recorded and replayed by two or more magnetic heads driven by a servo-mechanism. Typically, the servo-mechanism is designed such that a magnetic head is in contact with the advancing magnetic tape to record or replay the video signal. During recording, the video signal is recorded on the magnetic tape by the magnetic head then in contact with the magnetic tape. Since the magnetic head scans the magnetic tape in a helical manner, the television is recorded as diagonal tracks on the magnetic tape, with each track containing a field of the video signal.
A typical VCR is designed such that, whether recording or replaying, as soon as the magnetic head in contact with the magnetic tape traverses of the width of the magnetic tape, another magnetic head comes into contact to record the next field of video signal. Both the servomechanism and the magnetic tape contribute to the variability in the video signal provided by a VCR. In a VCR, a transient vibration is often caused at the beginning of each field by a magnetic head coming into contact with the magnetic tape. This transient vibration often lasts up to the duration of 25 horizontal lines. In addition, because of the variability inherent in the servomechanism, there are also variations in the time intervals between horizontal lines.
FIG. 1 shows the waveform 100 of a video signal under NTSC in the immediate vicinity of the horizontal synchronization pulse 102. FIG. 1 shows section 101, which encodes the picture elements ("pixels") at the end of the immediately preceding line, preceding horizontal synchronization pulse 102.
Horizontal synchronization pulse 102 comprises three waveform sections 102a, 102b and 102c. Waveform section 102a, having a duration of 1.5 microseconds (uS), is at a signal level known as the "blanking level." Waveform section 102b, having a duration of 4.7 uS, is at a signal level known as the "sync level." Thereafter, the signal returns to the blanking level (waveform 102c), before providing a burst 103 of the color subcarrier to allow the receiving circuit to synchronize to the phase of the color component of the video signal.
One method for a receiving circuit to synchronize with the television signal is by detecting the mid-point of the falling edge in the horizontal synchronization pulse, i.e. point 104 of waveform 100. A deviation from the expected location in time of this horizontal synchronization pulse is termed a horizontal phase error, or simply a phase error.